Life Sciences: Year In Review 2003

Paleontology

Among the more intriguing stories in paleontology during 2003 was the discovery of the dinosaur Microraptor gui, a small dromaeosaur from the Early Cretaceous Jiufotang Formation of Liaoning, China. Xing Xu of the Chinese Academy of Sciences and colleagues reported that the 77-cm (2.5-ft)-long animal, which lived between 124 million and 144 million years ago, had fully modern, asymmetrical feathers on all four limbs. Dromaeosaurs belong to the dinosaur subgroup called theropods, which were bipeds (with hind limbs adapted for locomotion) and flesh eaters (ranging from species as small as chickens to the huge Tyrannosaurus). Beginning in the late 1990s with the discovery of the first fossils of feathered dinosaurs, it became widely accepted that birds evolved from small light-boned theropod dinosaurs and that feathers originated in nonavian theropods. The question of how flight itself evolved, however, was not settled and continued to be debated. The arguments centred on two hypotheses—the arboreal theory, which suggested that flight arose in tree-dwelling animals through an intermediate gliding stage, and the competing cursorial idea, which suggested that flight evolved in fast-running ground-dwelling animals. If the four feathered limbs on M. gui were used for gliding, as the authors proposed, it would strengthen the arboreal theory for the origin of flight.

Over the years many types of evidence have been applied to determine the feeding habits of theropod dinosaurs. A report by Raymond Rogers of Macalester College, St. Paul, Minn., and co-workers on the large Late Cretaceous theropod Majungatholus atopus from the Maevarano Formation of Madagascar described heavily tooth-marked fossil bones in support of the idea that the animal defleshed other dinosaur carcasses as it fed. Majungatholus, which was as much as 9 m (30 ft) from nose to tail, lived about 70 million years ago. The investigators found well-gnawed bones both of plant-eating sauropods and of Majungatholus itself bearing marks that matched the characteristics of the latter animal’s teeth, which indicated that the dinosaur was a cannibal.

Paul Sereno of the University of Chicago, Jeff Wilson of the University of Michigan, and colleagues reported that while sifting through fossil remains collected years earlier from deposits along the Narmada River in western India, they found fossil bones belonging to a new species of Late Cretaceous dinosaur. Rajasaurus narmadensis was a 9-m (30-ft)-long dinosaur of the theropod family Abelisauridae that lived about 67 million years ago. Abelisaurs are Cretaceous theropods with short, high skulls. They were also known from South America and Africa, which like India were part of the ancient supercontinent Gondwanaland.

The caves and deposits at Sterkfontein near Johannesburg, S.Af., are among the richest fossil hominid sites in the world. A report by Timothy Partridge and co-workers from the University of the Witwatersrand, Johannesburg, and Purdue University, West Lafayette, Ind., described recently discovered hominid specimens—possibly of Australopithecus—from Jacovec Cavern and elsewhere at Sterkfontein and contended that the specimens are a surprising four million years old. Previous age estimates for the hundreds of hominids from Sterkfontein ranged from 1.5 million to 3.5 million years. The more ancient age values came from a recently developed technique, called cosmogenic burial dating, that was used on the buried sediments associated with the fossils. When sediment is on the surface, its minerals are bombarded by cosmic rays from space. This process continually produces unstable isotopes of beryllium and aluminum that have fairly long half-lives—about a million years. After the sediments are buried, the bombardment stops and the radioactive isotopes decay without further replenishment. By measuring the quantity of the isotopes that remain in a sample, scientists can determine the age at which the sediment and its content of once-living remains were buried.

Since the 1970s the origin of modern primates (euprimates) has been a subject of considerable debate, and a variety of scenarios have been offered to explain how and why their evolution occurred. Jonathan Bloch and Doug Boyer of the University of Michigan presented evidence from a well-preserved 56-million-year-old specimen of Carpolestes simpsoni from the Clarks Fork Basin of Wyoming that could resolve this debate. Previous phylogenetic analyses had concluded that the carpolestids are closely related to the euprimates. From their examination of the skull and foot bones of the skeleton, the most complete carpolestid found to date, the investigators inferred that this 30-cm (one-foot)-long animal—and, hence, that the ancestor of modern primates—lacked forward-facing eyes and convergent vision but had an opposable big toe and that it was a grasper adapted for feeding in terminal tree branches. This is in opposition to other hypotheses that suggested the ancestor of the euprimates was either a specialized leaper or a visually directed predator.

The very earliest tetrapods (vertebrates with limbs) date back to the Late Devonian, about 370 million to 354 million years ago. Until 2003 the nine genera described from that age were known only from North America, Europe, and Greenland, apart from a single fragmentary specimen found in Australia. Min Zhu and colleagues from the Chinese Academy of Sciences and the Natural History Museum, London, reported discovery of the first Late Devonian tetrapod fossil from Asia. Their identification of an incomplete left mandible from nonmarine sediments of the Ningxia Hui region of northwestern China indicated that tetrapods became quite widely dispersed in a relatively short time.

A study by Moya Smith of King’s College, London, and Zerina Johanson of the Australian Museum, Sydney, concluded that teeth evolved more than once in primitive fish. It formerly had been assumed that teeth evolved only once, in a fish ancestral to all vertebrates with jaws, the gnathostomes. In examining specimens of members of the Arthrodira, an advanced group of extinct predatory jawed fish called placoderms, the investigators found teeth made of dentine. Previously all placoderms had been thought to lack true teeth. If, as speculated, the arthrodires derived from toothless placoderms that were not ancestral to other fish groups, then teeth must have evolved independently in the two lineages.

Samuel Zschokke of the University of Basel, Switz., described an unusual specimen of Early Cretaceous fossil amber from Lebanon in which could be seen an individual thread of viscid (sticky) silk from a spider web. This specimen demonstrated that both the spider superfamily Araneoidea and the use of viscid silk in aerial webs date back at least 130 million years. This silk thread still bore dozens of the glue droplets that typify this type of arachnoid silk.

The gymnosperm ginkgo tree (Ginkgo biloba) is in some ways a living fossil, having existed at least since the Middle Jurassic Period 170 million years ago. Previously, there had been a gap of 100 million years in the ginkgo fossil record. In 2003, however, Zhiyan Zhou of the Chinese Academy of Sciences and Shaolin Zheng of the Chinese Ministry of National Land and Resources described a fossil from the Early Cretaceous Yixian Formation of China that fit near the middle of the gap, with an age of 121 million years. The specimen was found to have reproductive structures different from the Jurassic fossils but similar to the modern ginkgo, which showed that the morphology of this ancient tree had changed little over the past 100 million years.

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